It has become apparent that in future both in the case of stationary applications and also in the case of vehicles such as hybrid vehicles and electric vehicles battery systems are being used ever more frequently. In order to be able to meet the particular voltage requirements for a respective application and to be able to provide the power that can be made available, a high number of battery cells are connected in series. Since it is necessary for current that is provided by a battery of this type to flow through all battery cells and a battery cell can only carry a limited amount of current, battery cells are in addition often connected in parallel in order to increase the maximum current. This can be achieved either by providing a plurality of battery packs within a battery cell housing or by connecting battery cells externally.
FIG. 1 illustrates the principal schematic diagram of a conventional electric drive system, such as is used in electric vehicles and hybrid vehicles or also in stationary applications such as when adjusting rotor blades of wind power plants. A battery 10 is connected to a DC voltage intermediate circuit and said DC voltage intermediate circuit is buffered by a capacitor 11. A pulse-controlled inverter 12 is connected to the DC voltage intermediate circuit and sinusoidal voltages that are phase-offset with respect to each other for operating an electric drive motor 13 are supplied by said pulse-controlled inverter 12 to three outputs by way of in each case two switchable semi-conductor gates and two diodes. The capacity of the capacitor 11 must be sufficiently large in order to stabilize the voltage in the DC voltage intermediate circuit for a period of time in which one of the switchable semi-conductor gates is switched to conduct. In a practical application, such as an electric vehicle, a high capacity in the mF range is achieved.
FIG. 2 illustrates the battery 10 of FIG. 1 in a detailed block diagram. A plurality of battery cells is connected in series and optionally in addition in parallel in order to achieve a battery capacity and a high output voltage required for a respective application. A charging and disconnecting device 16 is connected between the positive pole of the battery cells and a positive battery terminal 14. Optionally, a disconnecting device 17 can in addition be connected between the negative pole of the battery cells and a negative battery terminal 15. The disconnecting and charging device 16 and the disconnecting device 17 comprise in each case a switch 18 or 19 respectively, which switches are provided for disconnecting the battery cells from the battery terminals in order to disconnect the battery terminals from the voltage supply. Otherwise, as a result of the high DC voltage of the battery cells that are connected in series, there is a considerable potential risk for maintenance personnel or the like. A charging switch 20 having a charging resistor 21 that is connected in series to the charging switch 20 is in addition provided in the charging and disconnecting device 16. The charging resistor 21 limits a charging current for the capacitor if the battery is connected to the DC voltage intermediate circuit. For this purpose, the switch 18 is initially left open and only the charging switch 20 is closed. If the voltage at the positive battery terminal 14 achieves the voltage of the battery cells, the switch 19 can be closed and if necessary the charging switch 20 can be opened. The switches 18, 19 and the charging switch 20 significantly increase the costs for a battery 10, since high demands are placed on their reliability and on the currents that they are to carry.